Existing adjustable back rests used in aircraft and vehicle seats always incorporate hydraulic piston-cylinder units in conjunction with mechanical springs for adjusting and locking the backrest in position. The springs restore the back rest to its starting position when it is unloaded. A push-button is connected via a Bowden cable to a lever which, when the push-button, which is mounted on the arm rest of the seat is pressed in, acts upon a releasing member of the hydraulic cylinder, with the result that the locking of the piston-cylinder unit is released and the piston rod of the unit can be displaced against the force of the springs. When the push-button is released, the piston-cylinder unit locks any further movement. In this manner a seat back can be adjusted and locked as desired in a predetermined angular range usually of up to 28.degree..
The hydraulic piston-cylinder unit, together with the spring system which is frequently very complicated and failure-susceptible, is very expensive. In spite of the complication, associated with not inconsiderable cost, in the use of these adjustment devices, hydraulic piston-cylinder units have been used exclusively. With this arrangement, the release force required on the push-button is only slight, not exceeding 20-30N. It can thus be operated by children. Moreover, the release mechanism must be capable of installation, together with other accessories such as ashtrays, call-button and light switches, within the restricted dimensions of about 40 mm width of the arm rest.
Starting from the consideration that, on the one hand a sufficient restoring force of the spring system is necessary and thus the force required for releasing the adjustment movement is inevitably correspondingly high, the main object of the present invention is to provide an adjustment and locking device which avoids the use of a hydraulic piston-cylinder unit and mechanical springs, which needs no more than the permissible release forces on the push-button and can be installed subsequently in existing seats without further modification of the seats themselves.